This invention relates to a long-lasting phosphor and, more particularly, to a long-lasting phosphor with improved phosphorescence brightness and lifetime.
Fluorescence is a phenomenon that a material emits visible ray when excited by an excitation source applied from outside. A fluorescent lamp, a discharge tube and a cathode ray tube (CRT) emit fluorescence. A material which emits fluorescence is called a phosphor. When light emitted by a phosphor lasts after stoppage of excitation for duration of time sufficient for the light to be perceived by the eye, i.e., about 0.1 second or longer, the light is called phosphorescence. A phosphor which has a long persistent phosphorescence lasting for several hours at room temperature is called a long-lasting phosphor or a light storage phosphor. As long-lasting phosphors, there are two types of prior art materials, namely a sulfide represented by ZnS:Cu and an Eu.sup.2+ -activated alkaline-earth metal aluminate RAl.sub.2 O.sub.4 (R being an alkaline-earth metal). The ZnS:Cu sulfide long-lasting phosphors have been practically used for several decades but they are disadvantageous in that their after-glow lasts for a relatively short duration of time, i.e., about three hours at the longest. Further, this type of phosphor has a fatal defect in that a decomposition reaction of ZnS+H.sub.2 O Zn+H.sub.2 S takes place under the coexistence of ultraviolet ray contained in the sunlight and moisture contained in the air, making the phosphor become black in color and, as a result, the after- glow characteristics significantly deteriorate within a relatively short period of time. For this reason, this type of phosphor has only limited applications such as a luminous watch and a night-time display of a location in a house.
On the other hand, recently developed Eu.sup.2+ -activated alkaline metal aluminate long lasting phophors (U.S. Pat. Nos. 5,376,303 and 5,424,006, Japanese Patent Application Laid-open Nos. 8-73845, 8-127772, 8-151573 and 8-151574) exhibit higher phosphorescence brightness, longer lifetime as well as better chemical durability and light resistance than the ZnS:Cu phosphors and, therefore, these aluminate phosphors are expected to have wide applications such as signs for preventing desasters, signs for indicating locations for keeping away from danger and decorations in addition to the existing applications for luminous watches and night time display in a house etc.
As applications of long-lasting phosphors are expanded, however, it has been found that the phosphorescence brightness of the prior art long-lasting phosphors is still not sufficiently high. Accordingly, it is desired for long-lasting phosphors to have further improved phosphorescence brightness and lifetime characteristics. It is also desired for long-lasting phosphors to have a characteristic of being excited promptly in a relatively short period of time.
It is, therefore, a first object of the invention to provide a novel long-lasting phosphor having a higher phosphorescence brightness and a shorter excitation time than the prior art aluminate long-lasting phosphors.
In a case where a long-lasting phosphor is used for decoration purposes such, for example, as decorating a signboard, it is desirable to use after-glow including as many colors as possible. The above described Japanese Patent Application Laid-open No. 8-151573 discloses that different colors can be imparted by combinations of the aluminate phosphor with dominant and auxiliary activators. In this laid-open application, however, the types of auxiliary activators available for use for imparting different colors are limited and therefore colors available are also limited within a relatively narrow range.
It is, therefore, a second object of the invention to provide a novel long-lasting phosphor which, while realizing a longer lasting phosphorescence and a higher brightness, is capable of providing a richer variety of colors in the emitted phosphorescence.
In a case where a long-lasting phosphor is used for manufacturing an ornament, since it is extremely difficult to form, e.g., a tube of a small thickness below 1 mm with the prior art aluminate long-lasting phosphor made by sintering, it is desired to provide a long-lasting phosphor which has overcome this problem.
It is, therefore, a third object of the invention to provide a novel long-lasting phosphor which is capable of producing an article whose shape is difficult to produce with a long-lasting phosphor made by sintering.